The Adams Arms Gas Piston System is an end user installed after-market product for Mil-Spec ARs. It will fit on any AR barrel with a .750" or 3/4" diameter at the shoulder where the front sight base sits. The Carbine version will work on barrel lengths from 10.5" to 16" that has an existing carbine gas system. The mid length version will work on barrel lengths from 16" to 18" that has an existing mid length gas system. The Rifle version will work on barrel lengths from 18" to 24" that has an existing rifle gas system.